Generally, a wind turbine includes a tower, a nacelle mounted on the tower, and a rotor coupled to the nacelle. The rotor generally includes a rotatable hub and a plurality of rotor blades coupled to and extending outwardly from the hub. Each rotor blade may be spaced about the hub so as to facilitate rotating the rotor to enable kinetic energy to be converted into usable mechanical energy, which may then be transmitted to an electric generator disposed within the nacelle for the production of electrical energy. Typically, a gearbox is used to drive the electric generator in response to rotation of the rotor. For instance, the gearbox may be configured to convert a low speed, high torque input provided by the rotor to a high speed, low torque output that may drive the electric generator.
A wind turbine typically includes many bearings that provide relative movement between adjacent parts in a relatively efficient, low-friction manner. For example, in most wind turbines a “main shaft” extends from the rotor and into the nacelle and is supported by one or more “main bearings.” Additionally, the gearbox in the nacelle that steps up the angular speed of the main shaft includes several bearings. Furthermore, the yaw control system that rotates the nacelle relative to the tower to turn the rotor into/out of the wind, and the pitch control system that rotates the blades about their longitudinal axis also include various bearings that allow for enhanced operation of the wind turbine.
Conventionally, these main bearings are configured as roller element bearings characterized by having a structural element (e.g., a ball bearing) disposed between the two components which are moving relative to one another. Roller element bearings fail for any number of reasons, but ultimately their life is limited by surface fatigue and wear. Such limited-life components require regular maintenance so as to avoid larger scale failure modes. The replacement parts and maintenance for such limited-life components increase the overall costs of operating a wind turbine. Accordingly, wind turbine and bearing manufacturers strive for improved or alternate designs that extend the operating life of the bearings.
Journal bearings are generally known in the art as having a long operating life. The main reason for this is that, unlike roller element bearings, journal bearings do not have any structural element disposed between the two relative moving components, but instead have only a fluid film disposed therebetween. Thus, the wear and fatigue issues associated with roller elements, as well as the costs associated with their replacement and maintenance, may be avoided. Consequently, journal bearings appear to provide an attractive alternative to roller element bearings. Additionally, journal bearings are designed to eliminate surface-to-surface contact and thus operate with even lower friction, which may further increase efficiency.
Due to the unpredictability of the wind, start-up and shut-downs that occur with wind turbines, and the resultant range of operating conditions, manufacturers have traditionally relied on roller element bearings. However, roller element bearings alone are sometimes insufficient for wind turbines that are subject to large bending loads caused by varying wind speeds and directions around the rotor swept area. These bending loads are transferred to the rotor shaft and reacted through the rotor main bearings. The bending loads are more dominant than the other shear and axial loads and are typically reacted to the static main frame through a pair of axially spaced predominantly radially loaded bearings. When the axial spacing is short the bearings are highly loaded and expensive. When they are spaced apart they are less expensive but require a more expensive longer main shaft.
To this end, it would be desirable to utilize journal bearings, optionally in combination with roller element bearings, in a wind turbine compound main bearing that is simple, small, and compact configuration and can stably support radial loads and axial loads as well as increase the operating life of the bearings and decrease the costs associated with replacement and maintenance.